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The letters stand for (Back Electro Motive Force).
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∙ 10y ago
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What is bemf in air conditioning?
no
How do you calculate DC link capacitors?
Its a very difficult subject to which most of my fellow designers estimate the ripple current in the caps and then select capacitors that can handle that current. The idea of voltage droop om most DC links can be forgotten because of other compensators such as a lower BEMF motor. The problem is to get as much life out of the caps as you can and that's to keep them cool as you can. Assume that every amp delivered from the DC like is an amp of ripple current and you should be OK.
Why current decreases when increase in viscous friction of dc motor?
I think your question could be rephrased as:"When viscous friction increases, why does currentdecrease?". Though the latter is not directly dependent on the former, or vice versa, in most motors this is the case. To understand exactly why this happens we need to look into the constructional features of a motor.All motors have a rotor (the spinning part) and a stator (the stationary part). In linear motors the part the slides along can be thought of as the 'rotor', and the non-moving base, as the 'stator'. The motor spins by causing like poles on the stator and rotor to oppose each other (different motors - AC, DC, brushless - do this in different ways). Read up about this on Wikipedia, HowStuffWorks, or just Google around. For maximum efficiency it is desirable to build the rotor and stator in such a way that there is very little space between the two. This allows a high degree of magnetic flux linkage/concentration between the rotor and stator. Between the rotor and stator is a small air gap. Air, like all fluids, has viscosity (a sort of 'stickyness', if you will). Viscous force appears between two sliding objects which have a liquid in between them. The closer the sliding surfaces, the greater is the viscous force. Viscous force also depends on the area of the surfaces, their relative velocity, and the liquid's properties. For a detailed derivation please refer to any text on fluid dynamics.Another important motor characteristic is back EMF (BEMF), which is induced in motor coils due to magnetic interaction. The faster a rotor spins, the greater the magnitude od back EMF. Back EMF, as its name suggests, has a polarity opposite to the applied (battery/power source) EMF. It is the back EMF which causes a reduction in motor current, since it partially cancels out the applied EMF : (Applied EMF-Back EMF)=(Effective EMF). Since the coils in the motor have unchanging resistance, a lower apparent EMF imples lower current.Now let us link these two phenomena:low speed-->low BEMF-->high currentalso;low speed-->low viscous frictionbut:higher speed-->higher BEMF-->lower currentand:higher speed-->higher viscous frictionI hope this clears things up.Rohit de Sa,Engineer,Mercedes-Benz,INDIA
